6b MORPHOLOGY AND EVOLUTION
phalanges is covered by obliqne chains. When the ridges are examined through a powerful lens they exhibit the orifices of the ducts of sweat glands.
The back of the hand and fingers, except the terminal phalanges, is covered with white, grey or black hairs, but the colour is not specific.
In the Hoolock Gibbon, for example, it may be black or white. There are no traces of callosities on the backs of the penultimate phalanges as in higher Apes. When the sides of the fingers are examined it is seen how there is a very sharp line of demarcation between the hairy dorsum and the hairless palm.
The foot: Like the hand the foot is long and narrow, and its digits have the same relative lengths as the fingers; but the hallux is thicker than the toes. The skin of the sole is glabrous and pigmented, its colour being the same as that of the palm in each species. The primitive pads are represented, as in the hand, by globular expansions of the terminal phalanges, but the parts over the heads of the metacarpal bones are full. Flexure lines are situated over the interphalangeal joints, roots of the toes and sole, but these vary in different species, as shown in fig. 20. The papillary ridges are arranged much as in the hand. The dorsum of the foot and toes, except the terminal phalanges, is hairy. It is, therefore, evident that the external characters of the hand and foot have much in common.
The head is compressed and the crown is flattened ; but the shape of the crown and the prominence of the supra-orbital crests are masked by a thick growth of hair (fig. 13). The position of the crests is indicated by long hairs, or by the anterior border of a band of white hairs.